Re: [Xen-devel] [PATCH v2 1/2] Resize the MAX_NR_IO_RANGES for ioreq server

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Jul 6, 2015 at 1:38 PM, Paul Durrant <Paul.Durrant@xxxxxxxxxx> wrote:
>> -----Original Message-----
>> From: dunlapg@xxxxxxxxx [mailto:dunlapg@xxxxxxxxx] On Behalf Of
>> George Dunlap
>> Sent: 06 July 2015 13:36
>> To: Yu Zhang
>> Cc: xen-devel@xxxxxxxxxxxxx; Keir (Xen.org); Jan Beulich; Andrew Cooper;
>> Paul Durrant; Kevin Tian; zhiyuan.lv@xxxxxxxxx
>> Subject: Re: [Xen-devel] [PATCH v2 1/2] Resize the MAX_NR_IO_RANGES for
>> ioreq server
>>
>> On Mon, Jul 6, 2015 at 7:25 AM, Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
>> wrote:
>> > MAX_NR_IO_RANGES is used by ioreq server as the maximum
>> > number of discrete ranges to be tracked. This patch changes
>> > its value to 8k, so that more ranges can be tracked on next
>> > generation of Intel platforms in XenGT. Future patches can
>> > extend the limit to be toolstack tunable, and MAX_NR_IO_RANGES
>> > can serve as a default limit.
>> >
>> > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
>>
>> I said this at the Hackathon, and I'll say it here:  I think this is
>> the wrong approach.
>>
>> The problem here is not that you don't have enough memory ranges.  The
>> problem is that you are not tracking memory ranges, but individual
>> pages.
>>
>> You need to make a new interface that allows you to tag individual
>> gfns as p2m_mmio_write_dm, and then allow one ioreq server to get
>> notifications for all such writes.
>>
>
> I think that is conflating things. It's quite conceivable that more than one 
> ioreq server will handle write_dm pages. If we had enough types to have two 
> page types per server then I'd agree with you, but we don't.

What's conflating things is using an interface designed for *device
memory ranges* to instead *track writes to gfns*.  Fundamentally the
reason you have this explosion of "device memory ranges" is that what
you're tracking isn't device memory, and it isn't a range.  If your
umbrella isn't very good at hammering in nails, the solution is to go
get a hammer, not to add steel reinforcement to your umbrella.

My suggestion is, short-term, to simply allow the first ioreq server
to register for write_dm notifications to get notifications, and
return an error if a second one tries to do so.

If it becomes important for a single domain to have two ioreq servers
that need this functionality, then we can come up with an internal Xen
structure, *designed for gfns*, to track this.  My suspicion is that
it will never be needed.

 -George

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